Subscription standing: each account's 5-hour and weekly windows, used and resetting, read where the provider says and inferred from the usage ledger where it does not #372

Part of #341, after the usage ledger (src/core/usage.ts) lands.

• Per subscription, keep its windows (5-hour, weekly): cap, used, remaining, resets at. Read from the provider where it exposes it (Claude Code's rate-limit messages already carry reset times — crew/limits.ts); infer from the ledger's tokens/cost over the window where it does not, and mark each number read or inferred.
• An operator setting: headroom kept for a person's interactive use (a share of each window the crew never spends).
• standing(db, subscriptionId, now) in src/core/subscriptions.ts (or a sibling module), plain data, tested against hand-calculated windows.
• Expose it on the existing /api/v1/repos/:owner/:name/usage read and the MCP usage tool (per subscription).